Along with development of international communication, submarine cable systems in which an optical fiber is used as a transmission line have been also enlarged. In a submarine cable system, a branching device installed on the bottom of the sea is used in order to connect a plurality of terminal stations installed on land with each other. The branching device connects transmission lines including a plurality of submarine cables with each other according to predetermined specifications such as a wavelength. On the other hand, communication devices that are used in a submarine cable system are becoming more highly functional year by year. For this reason, there are cases where engineering work for replacing a branching device to something more highly functional is needed after the system has become operational.
Further, there are cases where the specifications for connection between terminal stations connected to a branching device is changed after the system has become operational, as well as cases where a newly-installed terminal station is connected to the branching device. When such specification change or connection of a terminal station becomes necessary, there is a need to bring up the branching device to which the terminal station is connected from the bottom of the sea, and perform engineering work for making the specification of the branching device conform to the specification of a terminal station to be changed or newly installed. Then, at the time of engineering work of the branching device, it is necessary to halt the operation of the submarine cable system to perform replacement work of the branching device.
In relation to the present invention, patent literature 1 discloses a light signal branching device for use in a transmission line of a wavelength division multiplexing (WDM) light signal.